Hasn't Earth warmed and cooled naturally throughout history?
Yes. Earth has experienced cold periods (informally referred to as “ice ages,” or "glacials") and warm periods (“interglacials”) on roughly 100,000-year cycles for at least the last 1 million years. The last of these ice age glaciations peaked* around 20,000 years ago. Over the course of these cycles, global average temperatures warmed or cooled anywhere from 3° to perhaps as much as 8° Celsius (5°-15° Fahrenheit). It was partly through their attempts to understand what caused and ended previous ice ages that climate scientists came to understand the dominant role that carbon dioxide plays in Earth’s climate system, and the role it is playing in current global warming. Learn more here and here.
Over at least the past million years, glacial and interglacial cycles have been triggered by variations in how much sunlight reaches the Northern Hemisphere in the summer, which are driven by small variations in the geometry of Earth’s axis and its orbit around the Sun. But these fluctuations in sunlight aren’t enough on their own to bring about full-blown ice ages and interglacials. They trigger several feedback loops that amplify the original warming or cooling. During an interglacial,
- sea ice and snow retreat, reducing the amount of sunlight the Earth reflects;
- warming increases atmospheric water vapor, which is a powerful greenhouse gas;
- permafrost thaws and decomposes, releasing more methane and carbon dioxide; and
- the ocean warms and releases dissolved carbon dioxide, which traps even more heat.
These feedbacks amplify the initial warming until the Earth’s orbit goes through a phase during which the amount of Northern Hemisphere summer sunlight is minimized. Then these feedbacks operate in reverse, reinforcing the cooling trend.
*Correction. An earlier draft mistakenly said that the last of these ice ages ended about 20 thousand years ago. The glaciation phase peaked around that time.
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